Are these all OK?
I like its smell.
Its smell is good / nice.
Its smell stayed inside. (in the room)
Its smell is long-lasting.

Question

Are these all OK?
I like its smell.
  Its smell is good / nice. 
  Its smell stayed inside. (in the room)
  Its smell is long-lasting.

Answer

Actually, the apostrophy here is incorrect. Since "it's" is a contraction of "it is" or "it has", it cannot be placed at the start of those sentences. "Its" is possessive, which is what Pelin was trying to say. So don't use apostrophes here :)

Answer

Don't forget to put in apostrophes.

I like it's smell 
It's smell is good/nice -  it would be more natural to say' it smells nice'
It's smell lingers in the room (is a better way to say this.)
It's smell is long-lasting

Answer

Yes, all of these are correct :)

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